In a decision that improves Republicans’ shot at winning a Senate majority, Rep. Cory Gardner (R., Colo.) has decided to run against Democratic Sen. Mark Udall, Republicans familiar with the decision said. And to help clear the field for Mr. Gardner, a tea-party allied candidate has agreed to drop his own Senate bid.

Mr. Gardner, a second-term House member who is well regarded by party leaders, had in past months resisted repeated entreaties by the National Republican Senatorial Committee to run in a state that has been a crucial swing state.

But now, with the state looking increasingly ripe for a competitive race, Mr. Gardner is expected to announce his candidacy in the next few days, a GOP official said, confirming news that was first reported by the Denver Post.

Republican officials believe that will put another swing state in play and give them an improved opportunity to pick up the 6 seats they need to win a majority in 2014.

Republican officials believe that will put another swing state in play and give them an improved opportunity to pick up the 6 seats they need to win a majority in 2014.But now,with polls showing Mr. Udall increasingly vulnerable, the state looking increasingly ripe for a competitive race, Mr. Gardner is expected to announce his candidacy in the next few days, a GOP official said, confirming news that was first reported by the Denver Post.

“Gardner’s entry into the race is a game changer, and makes Colorado much more competitive,” said Jennifer Duffy, an analyst of Senate races at the nonpartisan Cook Political Report. Looking at the race’s impact on range of competitive Senate races across the country, Ms. Duffy said, “It expands the map, and this is what Republicans have to do. If you need six seats, you have to expand it as much as possible and hope you have the wind at your back.”

Republicans also may be able to avoid a messy primary because Ken Buck, a tea party allied candidate who was the party’s unsuccessful Senate nominee in 2010, had been running for Senate but his campaign office confirmed Wednesday that he had decided to drop out and run for Mr. Gardner’s House seat instead. Another GOP candidate, state legislator, Amy Stephens, was reportedly also planning to drop out of the Senate race, but her office didn’t respond immediately to requests for comment.

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